Datasheet

MAX8702/MAX8703
Dual-Phase MOSFET Drivers
with Temperature Sensor
8 _______________________________________________________________________________________
efficiency while allowing operation with a variety of
MOSFETs and PWM controllers. A UVLO circuit allows
proper power-on sequencing. The PWM control inputs
are both TTL and CMOS compatible.
The MAX8702 integrates a resistor-programmable tem-
perature sensor. An open-drain output (DRHOT) signals
to the system when the die temperature of the driver
exceeds the set temperature. See the Temperature
Sensor section.
MOSFET Gate Drivers (DH, DL)
The DH and DL drivers are optimized for driving mod-
erately sized high-side and larger low-side power
MOSFETs. This is consistent with the low duty factor
seen in the notebook CPU environment, where a large
V
IN
- V
OUT
differential exists. Two adaptive dead-time
circuits monitor the DH and DL outputs and prevent the
opposite-side FET from turning on until DH or DL is fully
off. There must be a low-resistance, low-inductance
path from the DH and DL drivers to the MOSFET gates
for the adaptive dead-time circuits to work properly.
Otherwise, the sense circuitry interprets the MOSFET
gate as “off” while there is actually still charge left on
the gate. Use very short, wide traces measuring 10 to
20 squares (50 to 100 mils wide if the MOSFET is 1in
from the device).
The internal pulldown transistor that drives DL low is
robust, with a 0.35Ω (typ) on-resistance. This helps pre-
vent DL from being pulled up due to capacitive coupling
from the drain-to-gate capacitance of the low-side syn-
chronous-rectifier MOSFETs when LX switches from
ground to V
IN
. Applications with high input voltages and
long, inductive DL traces may require additional gate-to-
source capacitance to ensure fast-rising LX edges do
not pull up the low-side MOSFET’s gate voltage, caus-
ing shoot-through currents. The capacitive coupling
between LX and DL created by the MOSFET’s gate-to-
drain capacitance (C
RSS
), gate-to-source capacitance
(C
ISS
- C
RSS
), and additional board parasitics should
not exceed the minimum threshold voltage:
Lot-to-lot variation of the threshold voltage can cause
problems in marginal designs. Typically, adding a
4700pF capacitor between DL and power ground,
close to the low-side MOSFETs, greatly reduces cou-
pling. To prevent excessive turn-off delays, do not
exceed 22nF of total gate capacitance.
Alternatively, shoot-through currents may be caused by
a combination of fast high-side MOSFETs and slow low-
side MOSFETs. If the turn-off delay time of the low-side
MOSFETs is too long, the high-side MOSFETs can turn
on before the low-side MOSFETs have actually turned
off. Adding a resistor of less than 5Ω in series with BST
slows down the high-side MOSFET turn-on time, elimi-
nating the shoot-through currents without degrading
the turn-off time (R
BST
in Figure 4). Slowing down the
high-side MOSFETs also reduces the LX node rise
time, thereby reducing the EMI and high-frequency
coupling responsible for switching noise.
Boost Capacitor Selection
The MAX8702/MAX8703 use a bootstrap circuit to gen-
erate the floating supply voltages for the high-side dri-
vers (DH). The boost capacitors (C
BST
) selected must
be large enough to handle the gate-charging require-
ments of the high-side MOSFETs. Typically, 0.1µF
ceramic capacitors work well for low-power applica-
tions driving medium-sized MOSFETs. However, high-
current applications driving large, high-side MOSFETs
require boost capacitors larger than 0.1µF. For these
applications, select the boost capacitors to avoid dis-
charging the capacitor more than 200mV while charg-
ing the high-side MOSFET’s gates:
where N is the number of high-side MOSFETs used for
one phase and Q
GATE
is the total gate charge speci-
fied in the MOSFET’s data sheet. For example, assume
C
NxQ
mV
BST
GATE
=
200
VV
C
C
GS TH IN
RSS
ISS
()
<






MAX8702
MAX8703
V
DD
BST
DH
LX
(R
BST
)*
D
BST
C
BST
C
VDD
INPUT
(V
IN
)
N
H
L
( )* OPTIONAL—THE RESISTOR REDUCES THE SWITCHING-NODE RISE TIME.
Figure 4. High-Side Gate-Driver Boost Circuitry